This invention relates to chelate resins and to method of preparing such resins.
Chelation may be defined as an equilibrium reaction between a metal ion and the functional groups of a chelating agent characterized by the formation of more than one bond between the metal and a molecule of the chelating agent. In this manner, chelating agents control metal ions by blocking the reactive site of the metal ion and preventing it from entering into its normal and oftentimes undesirable reactions. For this reason, chelating agents are often used in purification processes such as the control of undesirable metal ions in water or other liquids.
Chelate resins are normally solid chelating agents which have the ability to extract metal ions from a liquid without substantial structural alteration of the solid resin. The most effective chelate resins possess capacity to chelate with a large number of metal ions before the need for regeneration, i.e., preparing the resin for reuse by displacing the metal ions removed by the resin. They also advantageously exhibit a high porosity and are resistant to physical deterioration such as excessive swelling or shattering. Moreover, to obtain maximum benefit of the resin's properties, a spheroidal particle size is often desirable.
Heretofore, chelate resins have been conventionally prepared by the addition of chelate active functional groups to an insoluble resin matrix such as a cross-linked, vinyl aromatic polymer, e.g., a cross-linked polystyrene. A method for adding iminodiacetic acid chelate active groups to such resins by the sequential steps of halomethylation amination and carboxylation is disclosed in Ion Exchange, by F. Helfferich, published in 1962 by McGraw-Hill Book Company, New York. Alternatively, the iminodiacetic acid groups are added to such resins by the reaction of halomethylated resin with a nitrile containing amine and the hydrolysis of the reaction product. See, for example, U.S. Pat. No. 3,043,809 to Mattano. Unfortunately, such methods require numerous process steps, each of which require relatively exacting control to prevent unwanted side reactions. Moreover, the resins prepared by such methods possess relatively low chelate stability constants with may multivalent cations.
Several improved methods for preparing chelate resins have been proposed. For example, U.S. Pat. No. 3,228,920 to D'Alelio discloses chelate resins which can easily be prepared by reacting a compound having an active hydrogen and a coordination group such as glycine which reacts with a cross-linked polymer having a functional group such as oxirane or carboxyl halide to form a chelate resin.
Similarly, U.S. Pat. Nos. 3,310,530; 3,352,801 and 3,354,103 to White teach that sequestering agents having certain reactive groups, e.g., an epoxy, an NH group, active hydrogen and the like, can be attached to cross-linked polymers containing a coreactive group to form chelate type resins. Alternatively, cross-linking can follow the attachment of the sequestering group. Unfortunately, the chelate resins prepared by such methods possess relatively low chelate exchange capacities, thereby requiring frequent regeneration during use.
In view of the stated deficiencies of the prior art, it remains highly desirable to economically and efficiently prepare chelate resins having a high capacity for multivalent metal ions.